The present invention relates to a method and a device for controlling suspension performance in vehicles having hydropneumatic suspension devices and extremely variable axle load ratios, in particular on vehicles in which the front axle is subjected to a low, medium or high static load range, depending on the application of the vehicle, and the suspension device has double-action hydraulic cylinders between the suspended masses and unsuspended masses, the pressure lines of the cylinders being connectable to a pump, a pressure-regulating valve being installed in the pressure line to the annular spaces, the pressure-regulating valve constantly correcting the pressure in the annular spaces to a predefined pressure level.
German Patent Application 41 20 758 A1 describes a hydropneumatic suspension for vehicles having high axle-load differences, using double-action hydraulic cylinders whose cylinder spaces are connected to a first accumulator and whose annular spaces on the piston rod end are connected to a second accumulator, where a level-regulating valve regulates the level and a pressure-controlled valve steadily regulates a predetermined pressure ratio between the pressures of the first and second accumulators as a function of the load of the hydropneumatic actuators. The pressure-controlled valve is actuated by the pressure in the pressure line to the cylinder spaces as well as by the pressure in the pressure line to the annular spaces. Steady regulation is achieved as a function of the load of the hydropneumatic actuators. The annular space pressure is regulated as a function of load.
A simpler, more economical embodiment is described in German Patent 42 42 448 C1, the advantage being that it is possible to use load-sensing pumps. After changes in load and subsequent measures regulating the level up or down, static load changes are corrected. A pressure-regulating valve is used, maintaining a constant pressure level in the annular space spring circuit. With medium loads, however, the suspension becomes hard and occasionally uncomfortable.
An improvement in this situation with regard to comfort is achieved with the device according to German Patent Application 197 19 077 A1 by using a load-adjusted two-stage pressure-regulating valve as the pressure-regulating valve. This permits better adaptation of the suspension to the load cases. A change in the annular space pressure between a lower limit and an upper limit as a function of load is possible.
German Patent Application 197 19 076 A1 describes a device which allows an increase in load ratios without any increased stress on the parts and also improves driving comfort in the sense of a softer suspension. The regulating spring and the positioning spring of the pressure-regulating valve are coordinated to yield a load-dependent change in the annular space pressure which results in an increase in the spring constant at a low axle load with the possibility of making the spring constant softer in the medium load range to improve comfort. The functional advantages have been proven in experiments. However, with this type of device, the restricted manufacturing tolerances required for reproducibility of function and the unfavorable design size are unsatisfactory.
The speed of the pressure adjustment has proven to be inadequate for permanent suspension use.
Therefore, the object of the present invention is to create a control system in which the tolerance requirements are less stringent for economic manufacturing, a low installation volume is made possible and the required rapid pressure adjustment is implemented. The low load range of the front axle of the vehicle is understood to be the range of the suspended axle load which occurs when the vehicle is provided with a load such as a plow on its rear end. The high load range occurs when a loader or a similar device is attached to the front end of the vehicle. The medium load range occurs when there is no load on the vehicle.
Experiments have shown that a comfortable suspension design in the medium load state with a reduced axle load due to devices attached to the rear end, especially with the center of gravity projecting far outward, is associated with unacceptable vibration against the end stops, so that either an additional shock absorber function must be installed or the spring constant must be harder.
The object formulated here according to one exemplary embodiment of the invention is achieved by increasing the pressure in the annular spaces of the spring cylinders in the low load range on the front axle. In the medium load range, the pressure in the annular space is reduced for the purpose of increasing comfort, and in the low load range, the spring constant is made harder by increasing the annular space pressure.
In conjunction with the accumulator design, the control system may be coordinated so that the pressure in the annular spaces may remain constant for the medium and high load cases. On the other hand, however, it is also possible to increase the annular space pressure even with a higher load on the front axle.
The increase in annular space pressure may be adjusted to various values. These values are to be coordinated with the tractor size and the respective load states and should be on the order of magnitude of 20-40 bar in conjunction with the ring area of the spring cylinder.
The accumulator design may be coordinated so that the pressure value in the annular spaces may remain constant in both medium and high load ranges.
The device for implementing this method includes the use of a pilot valve which is controlled by the inlet pressure to the cylinder spaces and by the outlet pressure.
The pressure-regulating valve equipped with a servo-piston regulates an upper and lower pressure value. The pressure to be regulated is determined by the precontrol.
A 3/2-way valve may also be used as the pilot valve and is to be controlled electrically by a pressure sensor in the cylinder spring circuit. Another reversing function is to be provided in the case of the required hardening of the spring constant in the high load range.
The advantages of the present invention are achieved with a soft initial basic setting in the spring design which yields a good driving comfort in the medium load range and a harder coordination in the low load range, thus avoiding the problems of vibration against the end stops.